Existing optical devices for relative motion detection utilize pattern correlation techniques to determine relative motion between the device and a surface by capturing patterns characterizing the surface as the device passes over the surface (or equivalently as the surface moves past the device). Both the distance and the direction of the device movements are determined by comparing one pattern frame with the next. This technique typically detects intensity variations of shadows on surfaces; and its sensitivity and usability depends on the intensity contrast in the captured surface patterns. Relative motion sensors are used, for example, for computer pointer (e.g., mouse) control. Such pointers typically use optics to control the position of the pointer on the computer screen. More generally, optical navigation information can be used to compensate for distortion artifacts of curvilinear and rotational movement of the scanning device along a scan path.
U.S. Pat. Nos. 5,786,804, 5,578,813, 5,644,139, 6,442,725, 6,281,882 and 6,433,780 disclose examples of optical mice, other hand-held navigation devices, and hand-held scanners. These patents are incorporated herein by reference.
Typical existing optical navigation devices use light emitting diodes (LEDs) to obliquely illuminate the surface to be navigated (the “navigation terrain”) for the purpose of imaging the surface using a detector. Height variations on the surface, on the order of 5 to 500 micrometers (μm), cast shadows described by geometrical ray optics. The size and contrast of the shadow patterns depends in part on the type of surface through the size of the height variation. Typically, the detector is positioned to receive the reflection in the surface normal direction, and the angle between the surface and the incident light is typically selected to optimize the contrast of the shadow patterns, as is familiar from dark field imaging. Typical values for the angle of incidence are in the range from about 5 degrees to about 20 degrees.
Smooth surfaces such as whiteboard, high gloss paper, plastic, wood grain, or painted metal present functional challenges to typical current optical navigation devices. In general, smooth surfaces are those containing less mid spatial frequency and more high spatial frequency structures. To increase the signal level, high optical power is required for LED illumination resulting in typical current draws in excess of 30 mA.